Experimetal Ultraviolet Light (UVL) carcinogenesis in mice represents an excellent model to study tumor immunology. The majority of primaryneoplasms induced with this physical agent are immunologically rejected when transplanted to normal syngeneic recipients, but capable of progressive growth when transplanted to UVL-exposed animals. This observation has been extensively investigated and appears to be due, at least in part, to effects mediated by UVL directly on the immunologic apparatus. To date, UVL-induced tumors ahve mainly served as a tool to study this biological phenomenon. The aim of this proposal is to address a number of questions, crucial to the development of this model, using a reversal of the tact taken in our past investigations. Employing tumor specific immmuity as a tool, we propose to question: a) the clonal nature of UVL-induced tumors, b) the possibility that tumor-tumor or tumor-somatic cell hybridization may represent aa integral component of normal tumor progression, c) to further analyze the tumorigenic heterogeneity of cells within primary UVL-induced regressor phenotype tumors, d) to determine whether the gene(s) whih encodes for UVL-mediated transformation and the gene(s) which encodes for a particular TSTA is identical or at least linked, and 3) to further investigate the phenomenon that cloned lines derived from UVL-regressor phenotype tumors can be converted with normal splenocytes (induction, selection or both) to a virulent progressor which expresses a TSTA identical to the original cell line. The facilities and technical expertise needed to conduct these propsoed studies are somewhat unique in that a basic understanding of contemporary cellular immunology, tumor immunology, photobiology, somatic cell genetics and molecular genetics are essential. It is obvious that many aspects of experimental photocarcinogenesis have important human implications. UVL represents a well recognized and documented human carcinogen and its ability to modify certain human immune responses is just beginning to be appreciated.